Concentration ratio measurements via laser spectroscopic methods have been in use for many years. The most commonly studied elements have been carbon (13C/12C), distantly followed by hydrogen (D/H). The two main measurement methods are based on emission spectra and absorption spectra. Initially, quantitative determination of concentration ratios via the emission spectra of molecular transitions was inaccurate and not useful. At the same time, the absorption spectra lacked the resolution needed to characterize overlapping but isotopically different molecular transitions. This was mainly due to lack of power at monochromatic wavelengths. With the advent of lasers in the late 1960's, this limiting factor was eliminated, and high-resolution characterization of polyatomic species with isotopic substitution was possible. It was not until the completion of laser absorption studies in controlled laboratory settings that quantitative emission studies became useful.
Laser isotopic studies of carbon in methane and other short chain hydrocarbons continued for academic purposes until the late 1980's. In the 1990's, little scientific work was done in the field of carbon isotopic measurements in hydrocarbon gases. Since the 1990's, the academic focus has shifted to laser isotopic studies of inorganic polyatomic molecules. Commercialized applications of 13C/12C measurements have been used in medical research (measuring exhaled carbon dioxide), and in geological research for determining inorganic characterization of water and carbon in sandstone/mudstones, pyrite, sphalerite, galena and calcite. In the analysis of exhaled CO2, light emitted by a CO2 laser is used to measure isotope ratios. Inorganically bound isotopes of sulfur, oxygen, and hydrogen have also been studied with lasers for geological and environmental purposes.
Although much improved, current laser spectroscopic methods still possess some shortcomings. For example, in the context of a sample that contains two chemical species A and B having concentrations [A] and [B], respectively, concentrations are typically calculated from measurements of very small deviations of R=[A]/[B] ratio from the same ratio Rst in the reference (standard) sample. The most common application where such measurements are required is isotopomer abundance quantification. In this case, the deviation from standard is commonly expressed as
                              δ          [                      %            ⁢                                                  ⁢            o                    ]                =                                            R              -                              R                st                                                    R              st                                ×          1000                                    (        1        )            
The existing spectroscopic approaches to measure δ require precise separate measurements of the absorption lines for A and B with the subsequent numerical calculation of δ. This approach requires an extremely high accuracy of measurement because practically important δ ranges are between ˜1% to 0.1%. For example, in traditional approaches the required or desired measurement accuracy for [B] may be 10−4. In practice, making such precise measurements is extremely difficult due to small variations in temperature, pressure, and other external factors.
The most common tool for this type of measurements is a mass-spectrometer (MS). MS provides the required accuracy, but there are a number of shortcomings associated with this technology. Mass spectrometers are expensive, bulky and in general can not be used in the field. A sample preparation is required that can potentially affect the isotopic composition. Confusion between molecules or molecular fragments with similar masses is possible.
Infrared molecular absorption spectroscopy is considered as a viable alternative to MS, but few groups have succeeded in achieving the required accuracy even in laboratory experiments Current optical instrumentation for determination of isotopic composition is based on separate precise measurements of the strength of absorption lines corresponding to two isotopes with the subsequent numerical comparison. Hence, a small difference between isotopic compositions of the analyzed sample and the reference sample is determined as a difference between two large numbers (concentration ratios). Some of the issues adding to the error of such an approach are: the temperature and pressure dependence of the absorption line intensity; non-linearity of laser tuning; baseline distortions caused by spurious interference fringes and far wings of the irrelevant strong absorption lines; and isotopic fractionation in the sampling procedure.
Another problem with present laser spectroscopic techniques has been the detection of species with broad irresolvable absorption features, which is a characteristic of many polyatomic molecules. In such cases, a semiconductor laser usually can not be wavelength modulated with a swing sufficient to cover the whole absorption feature. Thus, detection of such molecules would require amplitude modulation of the laser radiation. The scattered and subsequently absorbed light creates an incoherent background, making low-level concentration measurements difficult.
Accordingly, there is a need for a simple method of accurately measuring small deviations in concentration ratios using laser spectroscopy. It is further desired to provide a laser spectroscopic method to detect minute concentrations of complex molecules.